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Chronic Fatigue Syndrome Back to Square 1

Chronic fatigue syndrome is not caused by a mouse retrovirus, according to a study initiated by the National Institutes of Health to settle what had become a contentious scientific question.

The long-awaited results, posted online Tuesday in the journal mBio, found no link between the illness, also called myalgic encephalomyelitis, and mouse leukemia retroviruses, including one called XMRV. Two earlier studies had identified higher levels of the viruses in patients with chronic fatigue syndrome. Later research did not confirm the finding, and scientists blamed laboratory contamination for the earlier results.

The N.I.H. asked Dr. Ian Lipkin, a virologist at Columbia, to investigate. Dr. Lipkin recruited in the effort scientists who initially reported the link to mouse retroviruses, and they serve as authors on the mBio paper.

In the study, none of the researchers reported finding mouse leukemia viruses in any of 293 blood samples, half from people with chronic fatigue syndrome and half from those without it.

An estimated one million people in the United States have the condition; many are severely disabled and homebound.

Dr. Lipkin said that he viewed chronic fatigue syndrome as a major illness and intended to use blood samples he had obtained to investigate the causes.

A version of this article appears in print on 09/18/2012, on page D6 of the NewYork edition with the headline: Vital Signs | Risks: Chronic Fatigue Syndrome Back to Square 1.